The BBO crystal is a non-linear frequency conversion crystal that is widely used in optical fields such as laser frequency doubling,frequency mixing and optical parametric oscillation.However,due to the large departure angle of the crystal during the quadrupling process,the resulting frequency doubled light is gradually separated from the fundamental frequency light,which reduces the quadrupling efficiency.The effect of the incident spot shape on the quadrupling efficiency of the BBO crystal is simulated for both circular and rectangular shapes during the 532~266 nm frequency doubling process.The results show that the use of a rectangular spot incident on the surface of the BBO crystal can extend the effective doubling aperture length,reduce the effect of the walk-off effect on the doubling light and significantly improve the quadrupling efficiency.